The present invention relates to a color cathode ray tube in which the amounts of horizontal and vertical deflection of a plurality of electron beams is individually controlled to correct coma caused by a deflection magnetic field, and convergence errors of the plurality of electron beams are suppressed to thereby obtain a good image display over the entire phosphor screen.
In a color cathode ray tube having at least an electron gun comprising a plurality of electrodes, a deflection device, and a phosphor screen (a screen having a phosphor film, hereinafter also referred to as a phosphor film or merely referred to as a screen), the following techniques have been known as means for reproducing a good image over the entire phosphor screen.
Japanese Patent Publication No. Hei 4-52586 discloses an electron gun emitting three inline electron beams in which a pair of parallel flat electrodes are disposed on the bottom face of a shield cup in such a manner as to be positioned above and below paths of the three electron beams in parallel to the inline direction and to extend toward a main lens.
U.S. Pat. No. 4,086,513 and its corresponding Japanese Patent Publication No. Sho 60-7345 disclose an electron gun emitting three inline electron beams in which a pair of parallel flat electrodes are disposed above and below paths of the three electron beams in parallel to the inline direction in such a manner as to extend from one of facing ends of one of a pair of main-lens-forming electrodes toward a phosphor screen, thereby shaping the electron beams before the electron beams enter a deflection magnetic field.
Japanese Patent Laid-open No. Sho 51-61766 discloses an electron gun in which an electrostatic quadrupole lens is formed between two electrodes and the strength of the electrostatic quadrupole lens is made to vary dynamically with the deflection of an electron beam, thereby achieving uniformity of an image over the entire screen.
Japanese Patent Publication No. Sho 53-18866 discloses an electron gun in which an astigmatic lens is provided in a region between a second grid electrode and a third grid electrode forming a prefocus lens.
U.S. Pat. No. 3,952,224 and its corresponding Japanese Patent Laid-open No. Sho 51-64368 discloses an electron gun emitting three inline electron beams in which an electron beam aperture of each of first and second grid electrodes is formed in an elliptic shape, and the degree of ellipticity of the aperture is made to differ for each beam path or the degree of ellipticity of the electron beam aperture of the center electron gun is made smaller than that of the side electron gun.
Japanese Patent Laid-open No. Sho 60-81736 discloses an electron gun emitting three inline electron beams in which a slit recess provided in a third grid electrode on the cathode side forms a non-axially-symmetrical lens, and an electron beam is made to impinge on the phosphor screen through at least one non-axially-symmetrical lens in which the axial depth of the slit recess is larger for the center beam than for the side beam.
Japanese Patent Laid-open No. Sho 54-139372 discloses a color cathode ray tube having an electron gun emitting three inline electron beams in which a soft magnetic material is disposed in fringe portions of the deflection magnetic field to form a pincushion-shaped magnetic field for deflecting the electron beams in the direction perpendicular to the inline direction of each electron beam, thereby suppressing a halo caused by the deflection magnetic field in the direction perpendicular to the inline direction.
FIG. 18 is a fragmentary sectional view showing an example of an electron gun for a color cathode ray tube using three in-line electron beams.
In FIG. 18, reference numeral 1 designates a first grid electrode (G1), 2 a second grid electrode (G2), 3 a third grid electrode (G3), 4 a fourth grid electrode (G4), 5 a fifth grid electrode (G5), 6 a sixth grid electrode (G6), 30 a shield cup, 38 a main lens, and K a cathode.
In this electron gun, the fifth grid electrode 5 is a focus electrode, the sixth grid electrode 6 is an anode, and the shield cup 30 is connected to the sixth grid electrode 6. In a cathode ray tube, the shield cup 30 is directed toward the phosphor screen.
FIGS. 19A and 19B are schematic sectional views of main parts for comparison of the construction of an electron gun depending upon the manner of applying focus voltages, FIG. 19A shows a fixed focus voltage type, and FIG. 19B shows a dynamic focus voltage type.
The electrode construction of the fixed focus voltage type electron gun shown in FIG. 19A is the same as that shown in FIG. 18, in which the same functional parts as in FIG. 18 are indicated by the same reference numerals.
In the fixed focus voltage type electron gun shown in FIG. 19A, a same focus voltage Vf1 is applied to electrodes 51 and 52 constituting the fifth grid electrode 5 thereof.
On the other hand, in the dynamic focus type electron gun shown in FIG. 19B, different focus potentials Vf1 and Vf2 are applied to two electrodes 51 and 52 constituting the fifth grid electrode 5, respectively. In particular, a dynamic focus voltage dvf superposed on Vf2 is applied to the electrode 52. Further, in the dynamic focus type electron gun, a portion of one electrode extends into the interior of another electrode as indicated at reference numeral 43, which has disadvantages in that its construction is complicated as compared with the electron gun shown in FIG. 19A, the cost is high, and its workability in assembly of the electron gun is inferior.
FIGS. 20A and 20B are explanatory views of focus potentials applied to the electron guns shown in FIGS. 19A and 19B. FIG. 20A shows a focus voltage waveform for the fixed focus voltage type electron gun, and FIG. 20B shows a focus voltage waveform for the dynamic focus voltage type electron gun.
In FIG. 20B, there are a fixed focus voltage Vf1 and a voltage having a waveform of a dynamic focus voltage Vf2 superposed on another fixed focus voltage Vf20. Therefore, in the dynamic focus voltage type electron gun shown in FIG. 19B, two high-voltage lead-in pins are needed at a stem of a cathode ray tube for supplying two focus voltages, and more consideration to insulation from other adjacent stem pins is necessary than in the fixed focus type electron gun. This poses problems in that a socket of a special construction is needed for a cathode ray tube installed in a TV set, and in addition to two fixed focus voltage sources, a dynamic focus voltage generating circuit is necessary, and additional set-up time is required for adjustment of the focus voltages in the assembly line of the TV set.
Further, in the color cathode ray tube provided with the in-line type electron gun of this kind, the center electron gun is on the tube axis, but the side electron guns are offset from the tube axis so that the side electron beams travel a greater distance in the deflection magnetic field than the center electron beam, receive a larger amount of action by the deflection magnetic fields, and create rasters of a horizontally and vertically larger size on the phosphor screen than the central beam.
As a result, the rasters created by three electron beams are not coincident with each other on the phosphor screen, resulting in convergence errors.
Further, in the color cathode ray tube of this kind, when the maximum deflection angle of the electron beams is fixed, the larger the size of the phosphor screen, the longer is a distance between the phosphor screen and the main lens of the electron gun, and hence the more is focus characteristics degraded by the mutual space-charge repulsion of electrons in this region.
Accordingly, if the distance between the main lens of the electron gun and the phosphor screen is made shorter with some means, fine electron beams can be obtained as in a cathode ray tube with a small-sized phosphor screen to enhance the resolution of the color cathode ray tube.
The shortening of the distance between the main lens of the electron gun and the phosphor screen increases the deflection defocusing and deteriorate the resolution at the periphery of the screen. Therefore, in the above-described prior art, there requires the measure for further raising the dynamic focus voltage so that the technical and cost-wise burdens on the side of the image display device employing the cathode ray tube such as the increased cost of the driving circuit and the improved high voltage breakdown capacity of a socket for a cathode ray tube.
Further, the depth of the present-day TV sets depends on the overall length of the cathode ray tube, and it is preferably short considering the TV set is a kind of a furniture. Moreover, the short depth of TV sets is preferable in terms of transport efficiency for TV set makers transporting a large number of TV sets.